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Description/Abstract

1. Extensive changes in marine communities in southwest Britain and the western English Channel have been recorded during the past 70 years.

2. Over the same period there was a climatic warming from the early 1920s, then a cooling to the early 1980s, with recent resumption of warming; the change in annual mean temperature was approximately ±0.5°C.

3. Marked changes occurred in plankton community structure; the distribution of both plankton and intertidal organisms was affected, with latitudinal shifts of up to 120 miles; there were increases or decreases of 2–3 orders of magnitude in abundance.

4. Warm water species increased in abundance and extended their range during periods of warming, while cold-water species declined or retreated; the reverse occurred during the period of cooling.

5. Climate change can influence marine communities by a combination of: direct effect on the organisms; effects mediated by biotic interactions; and indirectly through ocean currents.

6. From climate models that indicate rises of mean temperature of 2°C in the next 50 years, and from the observed changes, we can expect future latitudinal shifts of 200–400 miles in distribution of plankton, fish and benthos, with extensive restructuring of planktonic, pelagic and benthic communities.

7. Species common now in the Bay of Biscay will become common in the English Channel; those presently restricted to the western English Channel could colonise the central Irish Sea; changes in community structure could lead to lower abundances of infaunal benthos and fish.

8. To fully prove the effects of global warming, future changes in the marine biota must exceed those recorded in the 1950s and 1960s.